The first paragraph or two of any paper should be constructed with care, creating a path for both the writer and reader to follow. However, it is very common to adjust the introduction more than once over the course of drafting and revising your document. In fact, it is normal (and often very useful, or even essential!) to heavily revise your introduction after you've finished composing the paper, since that is most likely when you have the best grasp on what you've been aiming to say.
The introduction is your opportunity to efficiently establish for your reader the topic and significance of your discussion, the focused argument or claim you’ll make contained in your thesis statement, and a sense of how your presentation of information will proceed.
There are a few things to avoid in crafting good introductions. Steer clear of unnecessary length: you should be able to effectively introduce the critical elements of any project a page or less. Another pitfall to watch out for is providing excessive history or context before clearly stating your own purpose. Finally, don’t lose time stalling because you can't think of a good first line. A funny or dramatic opener for your paper (also known as “a hook”) can be a nice touch, but it is by no means a required element in a good academic paper.